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Merge pull request #47 from Bernardoow/data-in-session

Cookbook to put data in session
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psibi committed Aug 16, 2019
2 parents 1fa188b + e757bf1 commit e5170accbb69e74f201734da0d45d87041d62748
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  1. +97 −0 cookbook/data-in-session.md
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# Adding Data Type in Session

This is a example using simple YesodWeb Template.

### Json Step

My suggestion is to use Json string to save in session.

```
data MyResponse = MyResponse {
msg :: Text
} deriving (Show, Eq)
instance ToJSON MyResponse where
toJSON (MyResponse x) = object [ "msg" .= x ]
data Person = Person
{ age :: Int
}
deriving (Show, Eq)
instance FromJSON Person where
parseJSON = withObject "Person" $ \v -> Person
<$> v .: "age"
instance ToJSON Person where
toJSON (Person age) = object ["age" .= age]
toEncoding (Person age) = pairs ("age" .= age )
```

### Turning Json into Text

If we try to encode (from Data.Aeson) direct (encode $ person) an error is raised because encode return a ByteText and session must be Text.
So we need to work with this. I used encodeToLazyText :: ByteText -> Data.Text.Lazy and then toStrict :: Data.Text.Lazy -> Text.

```
setSession "object" ((toStrict $ encodeToLazyText $ person) :: Text)
```

### Turning Text into Json

When we want to do the way back we need to use fromStrict :: Text -> Data.Text.Internal.Lazy.Text, TL.encodeUtf8 :: Data.Text.Internal.Lazy.Text -> ByteString and then decode :: ByteString -> Maybe a

```
decode $ TL.encodeUtf8 $ fromStrict objectInSession :: Maybe Person
```

### Full ApiExample
```
{-# LANGUAGE NoImplicitPrelude #-}
{-# LANGUAGE OverloadedStrings #-}
module Handler.ApiExample where
import Import
import Data.Aeson (withObject, pairs, decode)
import Data.Aeson.Text (encodeToLazyText)
import qualified Data.Text.Lazy.Encoding as TL
import qualified Data.Aeson.Types as J
data MyResponse = MyResponse {
msg :: Text
} deriving (Show, Eq)
instance ToJSON MyResponse where
toJSON (MyResponse x) = object [ "msg" .= x ]
data Person = Person
{ age :: Int
}
deriving (Show, Eq)
instance FromJSON Person where
parseJSON = withObject "Person" $ \v -> Person
<$> v .: "age"
instance ToJSON Person where
toJSON (Person age) = object ["age" .= age]
toEncoding (Person age) = pairs ("age" .= age )
postExampleSetObjectInSessionR :: Handler Value
postExampleSetObjectInSessionR = do
mayeObjectInSession <- lookupSession "object"
case mayeObjectInSession of
Nothing -> do
let person = Person 29
setSession "object" ((toStrict $ encodeToLazyText $ person) :: Text)
sendStatusJSON status201 person
Just objectInSession ->
case (decode $ TL.encodeUtf8 $ fromStrict objectInSession :: Maybe Person) of
Just object -> do
liftIO $ print object
sendStatusJSON status201 object
Nothing ->
sendStatusJSON status400 (MyResponse { msg = "Some error happend in decode"})
```

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